This invention relates to the connection between landline telephone networks and two-way radio communication systems typically including a base station and at least one remote station, mobile or otherwise. More particularly, the invention relates to manual and automatic means for providing this interconnection in response to an initiating signal either from a distant telephone network subscriber (hereinafter referred to as the "telephone party") or the operator of a remote radio transceiver. In particular, the invention relates to interconnections between landline telephone networks and simplex radio communication systems.
In the past, most voice communications between landlines and two-way radio systems in the United States have been limited to amateur radio applications and to telephone and radio common carriers. Other services have not generally interconnected between radio systems and landline systems because of various problems associated with implementing these types of systems. In the case of telephone companies, ordinarily full duplex system have been provided. Such systems are, however, very limited in their application because of the lack of mobile-to-mobile capability, and since duplex frequencies are not generally available to most users, such as public safety, industrial, transportation and business radio and other licensees. Additionally, full duplex systems are relatively expensive.
Telephone patching arrangements for simplex radio systems have been utilized in the past, and examples of such prior art systems are disclosed in U.S. Pat. No. 3,557,312 issued to J. H. Vogelman on Jan. 19, 1971 and entitled "Remote Telephone Extension System", and in Canadian Pat. No. 729,289, issued on Mar. 1, 1966 to K. N. Jones, and entitled "Electronic Phone Patch Device". However, such prior art patching systems have incorporated considerable delays in switching between the receive and transmit modes, primarily as a result of the fact that such systems rectify the audio input signal and filter it by an RC bias filter wherein the capacitor has a substantial charging time constant which is in fact utilized to provide a sustain delay to prevent keying of the system on and off between syllables, the filtered signal constituting a control signal for the switching means. Such delays in signal recognition by the prior art systems have prevented them from being used by people who where not experienced with the operation of two-way simplex radio systems and did not have an understanding of the switching intervals involved. In such systems, attempts by inexperienced telephone parties to talk simultaneously with the transmission by the remote radio transceiver result in undesirable loss of information.
This problem is further aggravated by squelch noise which might be presented to or created by the base station receiver voice-operated-switch at the conclusion of a transmission by the remote radio operator. As a result of these factors, the delays in switching between transmit and receive modes or vice-versa in the prior art systems are typically anywhere between 150 and 500 ms, all of which is in addition to the 50 to 300 ms which might be required for the push-to-talk relay in the radio transceiver to operate in response to the switching signal. These delays render the prior art systems unsuitable for use by untrained personnel.
Furthermore, such prior art systems have not been fully automatic and, particularly, have not provided for completely automatic initiation of the phone patch interconnection by a remote radio operator while maintaining control of the interconnection by the radio licensee in accordance with FCC regulations. Therefore, base station operators are needed to complete such interconnections.
In addition, prior art patch systems are subject to being "locked up" by the base station receiver because of spurious signals on the radio communication channel other than those being transmitted from the remote radio operator involved. Thus, "skips", intermodulation and other spurious signals received by the base station receiver could lock out the telephone party and prevent him from gaining access to the base station transmitter.